This invention relates to a hydrothermal method of preparing cathodochromic sodalite, and particularly to such a method in which sodalite particles of extremely small size, e.g., 2 microns, can be prepared.
The term cathodochromic material, as used herein, refers to materials which can be colored by means of electron beam bombardment and in which coloration can be removed by means of heat or light. The use of sodalite as a cathodochromic material in dark trace cathode ray tubes is well known. Sodalites have the general formula Na.sub.6 Al.sub.6 Si.sub.6 O.sub.24 .2NaX where X is Cl-, Br-, and/or I-. Important features of any cathodochromic device, e.g., a dark trace cathode tube, are the contrast ratio, gray scale, and resolution of the cathodochromic material used therein. The contrast ratio of cathodochromic sodalite is very high, at least 20:1, and usually better than 50:1. The sensitivity of the sodalite to the electron beam is very high and shows gray scale with many steps. However, the resolution of the image lines on sodalite screens is limited by the size of the sodalite particles. Consequently, resolution of a cathodochromic device employing sodalite particles is improved by decreasing the size of the particles used. Also, small sized particles, i.e., less than 7 microns in diameter, are especially desirable since the sodalite material can then be more efficiently sensitized.
U.S. Pat. No. 3,773,540, issued Nov. 20, 1973, disclosed a hydrothermal method of preparing cathodochromic sodalite particles wherein the particles exhibited a size of about 4-6 microns in diameter. Although the hydrothermal method disclosed in the previously mentioned U.S. Patent is satisfactory for many applications; it is not desirable for preparing cathodochromic sodalite particles of extremely small size, e.g., about 2 microns in diameter. Furthermore, the hydrothermal method disclosed that, in general, the starting materials had to be heated above the critical temperature, about 356.degree. C, which is required for the formation of sodalite. Thus, it would be desirable to develop a method of preparing cathodochromic sodalite particles from which cathodochromic screens having improved resolution and sensitivity can be prepared.